A vehicle such as a hybrid vehicle having an engine and an electric motor as a driving source or an electric vehicle charges a battery mounted in a vehicle body, and generates driving force by using an electric energy from the battery. In general, a battery-associated power circuit is configured as a high-voltage circuit for handling high-voltage of 200 V or more. Further, in order to ensure safety, the high-voltage circuit including the battery is ungrounded structure electrically insulated from the vehicle body which is the ground reference potential point.
In the vehicle mounting an ungrounded high-voltage battery, an earth fault detector is provided so as to monitor a system in which the high-voltage battery is arranged, more specifically, an insulated condition (ground fault) between a main power system from the high-voltage battery to a motor and the vehicle body. In the earth fault detector, a system using a capacitor called as a flying capacitor is widely used.
FIG. 6 is a drawing showing an example of a circuit of the conventional earth fault detector of a flying capacitor system. As shown in FIG. 6, the earth fault detector 400 is connected to an ungrounded high voltage battery 300, and is a device for detecting an earth fault of a system having the high voltage battery 300. Herein, an insulation resistance between a positive side of the high voltage battery 300 and a ground is indicated as RLp, an insulation resistance between a negative side thereof and the ground is indicated as RLn.
As shown in FIG. 6, the earth fault detector 400 includes a detection capacitor C1 operating as a flying capacitor. Further, the earth fault detector 400 includes four switching elements S1 to S4 around the detection capacitor C1 so as to switch measurement paths and to control charge and discharge of the detection capacitor C1. Furthermore, it includes a switching element Sa so as to sampling a measurement voltage corresponding to a charge voltage of the detection capacitor C1.
In the earth fault detector 400, a measurement operation of V0 measurement period→Vcln measurement period→V0 measurement period→Vc1p measurement period as one cycle is repeated. In those periods, the detection capacitor C1 is charged with a voltage of a measurement target, and then a charge voltage of the detection capacitor C1 is measured. Also, in order to perform a next measurement, the detection capacitor C1 is discharged.
In the V0 measurement period, a voltage corresponding to a voltage of the high voltage battery is measured. For this reason, the switching elements S1 and S2 are turned ON, the switching elements S3 and S4 are turned OFF, and then the detection capacitor X1 is charged. That is, as shown in FIG. 7A, the high voltage battery 300, a resistance R1, and the detection capacitor C1 become a measurement path.
When measuring the charge voltage of the detection capacitor C1, as shown in FIG. 7B, the switching elements S1 and S2 are turned OFF, the switching elements S3 and S4 are turned ON, and the sampling is performed in the controller 420 while the switching element Sa is turned ON. Thereafter, as shown in FIG. 7C, the switching element Sa is turned OFF, and then the detection capacitor C1 is discharged so as to perform next measurement. When measuring the charge voltage of the detection capacitor C1, an operation when discharging the detection capacitor C1 is the same that in the other measurement period.
In the Vcln measurement period, a voltage reflecting the effect of the insulation resistance RLn is measured. Therefore, the switching elements S1 and S4 are turned ON, the switching elements S2 and S3 are turned OFF, and the detection capacitor C1 is charged. That is, as shown in FIG. 8A, the high voltage battery 300, the resistance R1, the detection capacitor C1, the resistance R4, the ground, and the insulation resistance RLn become a measurement path.
In the Vc1p measurement period, a voltage reflecting the effect of the insulation resistance RLp is measured. Therefore, the switching elements S2 and S3 are turned ON, the switching elements S1 and S4 are turned OFF, and the detection capacitor C1 is charged. That is, as shown in FIG. 8B, the high voltage battery 300, the insulation resistance RLp, the ground, the resistance R3, the resistance R1, and the detection capacitor C1 become a measurement path.
It is known that (PLp×RLn)/(RLp+RLn) can be obtained based on (Vc1p+Vcln)/V0 calculated from V0, Vc, Vcln, and Vc1p obtained in those measurement periods. For this reason, the controller 420 in the earth fault detector 400 can get the insulation resistances RLp and RLn by measuring V0, Vcln, and Vc1p. Further, when the insulation resistances RLp and RLn becomes equal to or lower than a predetermined judgment reference level, it is judged that the earth fault is generated, and then an alarm is outputted.
Further, in a Patent Literature 1, the earth fault detector 440 having a circuit configuration as shown in FIG. 9 is suggested. In the earth fault detector 440, a switching state of each of the measurement periods is the same as the earth fault detector 400.
Patent Literature 1: JP 2009-281986 A